This invention relates to a photoreceptor composition useful in electrophotography. More particularly, this invention relates to a electrophotographic receptor composition comprising, in sequence, a conductive base layer, a carrier transport layer, a carrier generation layer, and a surface protective layer characterized by having a transmissivity not exceeding 50% for light having a wavelength of 405 nm or less.
In recent years, there has been extensive research on organic photoconductive substances as photosensitive materials for use as photoreceptors in electrophotography. A photosensitive material using an organic photoconductive material has many advantages in terms of properties such as flexibility, heat stability, film forming properties, transparency and cost. However, such materials have disadvantages in terms of darkness resistance and light sensitivity as compared with conventional photosensitive materials using an inorganic photoconductive substance such as selenium or the like. To overcome these disadvantages, photosensitive materials have been constructed using organic photoconductive substances which are formed by fabricating the photosensitive portion of the photoreceptor as a laminate of separate functional layers. The layers ordinarily comprise a layer contributing mainly to carrier generation, and a layer contributing mainly to retention of surface carriers in a dark place and to carrier transportation upon illumination of the photoreceptor. Such organic photoconductive substances are advantageous in that they are easy to form into a film. By selecting and using a material suitable to the respective function of each layer, overall electrophotographic characteristics of the photoreceptor may be improved.
This laminate-type photoreceptor is usually prepared by laminating a carrier generation layer containing an organic carrier generation material and a carrier transport layer containing an organic carrier transport material to a conductive base. Electrophotographic image formation using such a photoreceptor may be achieved, for example, using a Karlson method. Image formation according to this method is carried out by electrification of the photoreceptor by corona discharge applied to it in a dark place, formation of electrostatic latent images of letters, figures, and the like by exposing the surface of the photoreceptor to light, development of the electrostatic latent images formed with a toner, and transference and fixation of the developed toner image to a substrate such as paper or the like. After a toner image has been transferred, the photoreceptor is subjected to a process of removal of electrification, removal of residual toner, and removal of electrification by light before it is reused.
In the above-mentioned image formation method, a negative electrification method is typically employed to electrify the photoreceptor. This is disadvantageous because a significant amount of ozone is generated in a negative corona discharge, and the surface of the photoreceptor when electrified is strongly oxidized by ozone, causing deterioration of the photoreceptor itself or of other equipment. It would therefore be advantageous if a positive electrification method is employed in conjunction with a laminate-type photoreceptor, as the corona discharge is stable, and only a small amount of ozone is generated. In addition, a suitable developing agent is easy to produce in the case of positive electrification, as compared with the situation where negative electrification is employed. However, suitable organic carrier generation and transport materials necessary to produce a photoreceptor having the above-mentioned laminated functional layer design and to which a positive electrification method may be applied have not yet been found.
To make it possible to use a photoreceptor in conjunction with a positive electrification method, a method of formation of a single layer by mixing carrier generation and carrier transport materials, as well as a method of formation of a carrier generation layer on a carrier transport layer have been considered. However, it has been found that the former method has drawbacks such as low carrier-accepting capacity and lack of repeating characteristics. The latter method also is disadvantageous in that it is difficult to form a carrier generation layer of thickness not exceeding 1 micron, preferably not exceeding 0.3 micron, without changing the properties of the carrier transport layer.
In addition, in recent years it has become required that organic material-based photoreceptors exhibit durability equal to that of photoreceptors employing selenium. However, it has been found to be very difficult to satisfy such durability requirements with photoreceptor materials prepared by depositing a thin carrier generation layer onto a carrier transport layer. Several methods have been proposed to improve the durability of organic material-based photoreceptors by applying a surface protective layer having excellent abrasion resistance and light transmitting properties on a carrier generation layer. In particular, the use of tetraethyl silicate or a fluorine-containing comb-type polymer as the main component have been proposed.
However, in many cases, the surface protective layer is transparent in a region of all wavelengths of light, and transmits light of such a wavelength that even the carrier generation layer does not absorb it. If a photoreceptor with the above-mentioned surface protective layer is exposed to light of a fluorescent lamp for an extended period of time, the carrier generation material is fatigued by strong light of wavelength near 405 nm and the fatigued carrier generation material does not recover its characteristics unless allowed to stand in a dark place for several hours.
It is the object of this invention to provide an electrophotographic photoreceptor composition which comprises organic materials, may be used in conjunction with a positive electrification method, and which exhibits enhanced resistance to fatigue from exposure to light such as fluorescent light.